First , a little back-story, since it might contain my problem. I constructed a new computer about a week ago: new motherboard, processor, etc. Since it does not have an integrated video card, I moved the card from my old machine (an Emachine T3256) into it. I also simply swapped over my HD (running Windows XP at the time). Everything ran smoothly. I then upgraded my HD to an SATA 300 Gig (everything went smoothly with the switch) and installed Ubuntu on my 160 Gig IDE (the old one). I went to put the old hard-drive back in the old computer, but it kicks back the error message: DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER I checked the HD on my new system, and it boots fine when hooked up to the new motherboard. I checked the BIOS on the Emachine, and it recognizes the Hard drive is there. It just wont boot from it. Can anyone help me out with why? Also: I attempted to re-install a Windows OS on the 160 as well, and got the same error message.